This invention relates to a travelling wave motor, e.g. a compact ultrasonic motor using ultrasonic oscillation and having an electro-mechanical transducer for generating a travelling-wave and a movable member driven by the travelling wave.
One known ultrasonic motor is a standing-wave type motor which employs a Langevin oscillator as a driving source. Such a motor is disclosed in U.S. Pat. No. 4,019,073. Another known ultrasonic motor is a travelling-wave type motor employing a travelling wave generated on a stator for driving a rotor provided on the stator. The travelling-wave motor generates smaller wear in the friction-transmission surface between the stator and the rotor, and can more easily be driven in the reverse rotation direction by comparison with the standing-wave type motor. Such a travelling-wave motor is disclosed in U.S. Pat. No. 4,513,219 (Katsuma et al), U.S. Pat. No. 4,562,374 (Sashida) and EP-A-169,297 (Tokushima).
FIG. 2 represents one example of a traveling wave generation principle in the travelling-wave motor. A reference numeral 1 denotes a piezoelectric transducer consisting of piezoelectric ceramic, piezoelectric crystal, which is polarized at equal intervals by a width b with the adjacent polarizations opposite each other in direction as illustrated. An electrode 2 is formed on each piezoelectric transducer through evaporating or plating a conductive material such as silver, nickel or the like, and these electrodes are connected by signal lines 10, 11 each to have a high frequency voltage impressed thereon from a different signal source. Then, a void c in width is provided between electrode groups connected by the signal lines 10, 11 each. In this case, the void c in width may have nothing of presence of the polarization and the electrode. Here, the distance between centers of the electrodes across the width c will be denoted by a for the convenience of description. A mechanism of the travelling wave generation will be described with reference to FIG. 2 and reference characters given therein. An inflected vibration wave consisting of travelling wave and regressive wave may be expressed by the following equation from taking the midpoint of an electrode portion as a reference. EQU A sin (.omega.t-kx)+A sin (.omega.t+kx) (1)
Here, (a) indicates a so-called standing wave. Then, an inflected vibration wave on an electrode portion may be expressed as follows.
B sin {.omega.t-k(x+a)+.phi.}+B sin (.omega.t+k(x+a)+.phi.}(2)
were
K=.omega./.nu.=2.pi./.lambda. PA1 .lambda.: wavelength, .phi.: phase difference angle
In Eq. (2), where EQU -ka+.phi.=.alpha..pi. EQU ka+.phi.=.beta..pi. (3)
then Eq. (2) may be expressed as: EQU B sin (.omega.t-kx+.alpha..pi.)+B sin (.omega.t+kx+.beta..pi.)(4)
Accordingly, the inflected vibration wave excited by 1, 2 may be expressed in a type having Eqs. (1) and (4) put together. Here, if a condition for the presence of a traveling wave only is considered from the development of Eq. (4), it is understood that it comes in the case where .alpha. is an even number and .beta. is an odd number. Here, a and .phi. expressed by equations of .alpha. and .beta. may be given as follows from Eq. (3). ##EQU1## and thus the travelling wave component only is present when a and .phi. are satisfied each concurrently. If, for example, the case where ##EQU2## is considered, then Eq. (1)+Eq. (2) will be: EQU A sin (.omega.t-kx)+A sin (.omega.t+kx)+B sin (.omega.t-kx)-B sin (.omega.t+kx) (6)
Here, if amplitudes A and B of a high frequency voltage signal generated from a driving circuit are A=B, then Eq. (6) will be 2A sin (.omega.t-kx), and thus it is understood that the travelling wave component only remains. Further, for reversing drive the regressive wave component only will be made to remain, therefore and B in Eq. (5) will be inverted, thus .alpha. and .beta. being odd and even respectively. When considered with reference to 1 practically, a phase of the signal to be applied to 2 may be shifted 180.degree. as compared with the case of forward drive.
FIG. 3 represents a principle on which the travelling-wave motor runs according to a travelling wave component. A reference numeral 3 denotes a vibrator part, which may generate an inflected vibration as the piezoelectric vibrator is bonded to an elastic member. Now, when a travelling wave is generated rightward on the principle shown in FIG. 2, one spot on the surface of the vibrator part 3 draws a leftward elliptic path, therefore a rotor part 6 moves counter to the direction in which the travelling wave goes. The above is so reported in NIKKEI MECHANICAL (Sept. 23, 1985) and others, and a detail description on one spot on the surface of the vibrator part 301 drawing an elliptic path is also given therein.
Katsuma et al. and Sashida disclose a travelling-wave motor employing a Ling type piezoelectric member. FIG. 4 shows one of this type travelling-wave motor. This type of travelling-wave motor essentially consists of an annular vibrating body 403 and movable body 405 provided thereon. The vibrating body has as annular piezoelectric vibrator 404 thereon. The vibrating body 403 is fixed to a base 402 through a supporting mechanism 406. On the annular piezoelectric vibrator, a gap having a length of half of the arc of an electrode is provided between two electrode groups. The travelling wave is brought about by applying a AC signal having a phase difference of 90.degree. to the two groups.
Another type of travelling-wave motor employing a disk-shaped piezoelectric member is disclosed in the European Patent of Tokushima. FIG. 5 shows this type of travelling-wave motor. In the figure a stator is constituted by a disk-shaped resilient vibrating body 503 having a toothlike circular projection. The vibrating body 503 has a disk-shaped piezoelectric vibrator 504 thereon. A movable body 505 is provided on the projection of the vibrating body and has a central shaft to act as a rotational guide. A pressure-regulating device is provided on the central shaft for effecting a suitable contact pressure between the vibrating body and the movable body so that the travelling wave component can be efficiently transmitted to the movable body. The vibrating body is supported and fixed on two circular projections formed on a base. The disk-shaped piezoelectric vibrator consists of a plurality of electrodes arranged in such a manner that an electrode pattern 601a divided at equal intervals is provided on almost semicircular portion of one side plane of it as shown in FIG. 6 and an electrode pattern 601d divided at equal intervals likewise is provided on remain almost semicircular portion through blank areas 601b and 601c left in 0.5 pitches and 1.5 pitches on opposite end portions. The piezoelectric vibrator is polarized so that the adjacent electrode patterns is in the counter direction.
For mounting in this case, almost semicircular metallic plates, for example, are bonded conductively at every almost semicircular electrode blocks to homopolarization electrically, and signals 90.degree. different in phase are impressed thereon through two lead wires, thereby generating a travelling wave.
In a travelling-wave motor of the type described above, if its construction includes an annular vibrating body, the travelling wave is significantly damped to a certain degree due to the required supporting structure since the flexure mode travelling wave which has been excited by the piezoelectric vibrator has no nodal point of oscillation. As a result of this, the electro-mechanical transducing efficiency is low.
If the construction of the travelling-wave motor includes a disk-type of vibrating body, there is the advantage that the vibrating body can be fixed and supported by the base at two places in the radial direction of the base because the vibrating body is excited in a secondary oscillation mode in the radial direction of the vibrating body, however, in the case of a thin and small sized travelling-wave motor which is an object of the invention, a deterioration of efficiency is also quite unavoidable according to a dispersion of node positions and dimensions of the supporting area and force, and since a driving frequency exceeds 100 kHz for excitation in the secondary vibration mode, a deterioration of circuit efficiency may result.
If the piezoelectric element having the electrode patterns shown in FIG. 4 is used, one standing wave is excited on one semicircular portion, a standing wave 90.degree. different in phase therefrom is excited on the other semicircular portion, the standing waves different each other are propagated to the semicircular portions on the counter side mutually, and the two standing waves are thus synthesized to generate a travelling wave, therefore a source for generating the two standing waves comes one-sided to the semicircle, and thus a uniform travelling waves cannot be excited. As one quantitative data indicating such phenomenon, for example, some peaks called "spurious" peaks can be observed other than a rise at the resonance point in frequency-amplitude characteristics. Consequently, an electric-mechanical transduce efficiency of the motor detoriorates.